The field of the invention relates to engine air-fuel ratio control during catalyst desulfurization, and more particularly to operating some cylinder groups lean and others rich.
Engines can increase exhaust component temperatures by operating with some cylinders at a lean air-fuel ratio and other cylinders at a rich air-fuel ratio. When the gas streams of lean and rich gasses meet in the exhaust system and mix, an exothermic reaction occurs to generate heat. This reaction can be improved by having a catalyst in the exhaust. The mixture air-fuel ratio can be maintained at the stoichiometric ratio by providing feedback air-fuel ratio control based on a sensor in the exhaust manifold, which is upstream of the catalyst as shown in U.S. Pat. No. 4,089,310.
The inventors herein have recognized a disadvantage with the above approach. In particular, when trying to de-sulfate the catalyst, the oscillation of the overall exhaust air-fuel ratio may be insufficient. In particular, since the feedback from the exhaust manifold sensor causes oscillations based on the ratio of the mixture upstream the catalyst, control of the oscillations is performed irrespective of the conditions in the catalyst or the conditions downstream of the catalyst. Further still, if there are multiple catalysts in the exhaust system, control of the oscillations based on an exhaust manifold sensor may provide no oscillations in the air-fuel mixture entering catalyst downstream of the first catalyst (due to the filtering effect of the first catalyst on the exhaust air-fuel ratio). As such, downstream catalysts that need to be decontaminated, may received exhaust air-fuel mixtures without sufficient oscillations to effectively remove sulfur, or other contaminants.
The inventors herein have also recognized a disadvantage with DE 199,23,481. Using the system of this reference, the oscillation of the exhaust gas mixture can be provided by adjusting either the fuel injection amount or the air amount to all of the cylinders based on a sensor located downstream of the catalyst. However, in either case, adjustment in this way may not maintain the catalyst temperature at a necessary decontamination temperature. In other words, when operating all of the cylinders around stoichiometry, exhaust gas temperature may fall too low and decontamination can become inefficient since there is little to no exothermic reaction (i.e., all cylinders are either lean or rich).
Disadvantages with prior approaches are overcome by a method for controlling an engine having a first and second group of cylinders, both of which are coupled to an emission control device. The method comprises operating the first group on average at a first lean air-fuel ratio; operating the second group at a second air-fuel ratio; and adjusting said second air-fuel ratio based on a condition in or downstream of the emission control device by controlling fuel injected into the second group to cause a mixture air-fuel ratio of a mixture of gasses from the first and second group to oscillate around a predetermined air-fuel ratio.
By adjusting the second air-fuel ratio via fuel injected into the second group to cause a mixture air-fuel ratio of a mixture of gasses from the first and second group to oscillate around a predetermined air-fuel ratio, it is possible to minimize cylinder torque oscillations. Further, by taking into account either the conditions in or downstream of the catalyst, more efficient sulfur removal is possible.
Note that the result is that the fuel to the rich cylinders is adjusted differently than the fuel to the lean cylinders so that a mixture air-fuel ratio oscillates with minimal torque imbalance. The difference in adjustment may be an adjustment only to the rich cylinders based on the downstream sensor so that the mixture oscillates about stoichiometry, or both may be adjusted, but a larger adjustment is made to the rich bank. Any remaining torque imbalance can be handled by spark retard on the rich cylinder, if desired.
In an alternate embodiment, air added to the lean cylinder group is primarily adjusted to oscillate the mixture air-fuel ratio.